1. The Field of the Invention
The invention relates to endovascular medicine, and more particularly, to systems and methods for aneurysm treatment and selective vessel occlusion.
2. The Relevant Technology
Cerebrovascular disease encompasses a broad spectrum of disorders, including intracranial aneurysms. Unruptured intracranial aneurysms have a prevalence of approximately 3.6 to 6% in the U.S. population and have an estimated annual rate of rupture between 10-28 per 100,000. Most individuals with aneurysms remain asymptomatic. Multiple risk factors for the development of intracranial aneurysms include: smoking, hypertension, positive family history and cocaine use. A number of inherited disorders have also been associated with the development of intracranial aneurysms. Ruptured intracranial aneurysms are the most common cause of non-traumatic subarachnoid hemorrhage (SAH). SAH secondary to aneurysm rupture is a potentially lethal event and carries a 50% morbidity-mortality rate.
An aneurysm is an abnormal localized dilation of a vessel. Aneurysms most frequently occur at sites of arterial bifurcation and are most commonly found in the brain. Pathologically, aneurysms have an absent or fragmented internal elastic membrane. Intracranial aneurysms are classified as saccular, fusiform, dissecting or false. Approximately 90% of intracranial aneurysms are saccular and are described by size, contour, orientation, location and neck size. Select unruptured and all ruptured intracranial aneurysms require either surgical or endovascular intervention.
Surgical intervention had long been the gold standard of care for the management of intracranial aneurysms and involves the placement of a clip across the aneurysm neck. A recent meta-analysis reviewing the risks of surgical repair found an overall mortality rate of 2.6 percent and a permanent morbidity rate of 10.9 percent. Endovascular treatment of intracranial aneurysms has developed over the last two decades. The procedure most commonly involves the insertion of a “coil” of wire into the aneurysm. The coil is delivered to the aneurysm through catheters, which are placed and guided through arteries. The Guglielmi Detachable Coil (GDC) pioneered the field of endovascular treatment of intracranial aneurysms and involved electrolytic detachable platinum coils that were placed directly into the fundus of the aneurysm via a microcatheter and detached from a stainless-steel micro-guidewire by an electrical current. Studies suggest that endovascular treatment may be associated with less procedural morbidity and mortality than conventional surgical techniques as well as reduced recovery time and earlier return to normal functioning.
Although endovascular techniques have created a paradigm shift in the management of intracranial aneurysms, the technique still has many limitations. This is particularly evident in the treatment of wide-necked, dissecting and fusiform aneurysms. Until recently, wide-necked aneurysms (defined as an aneurysm with a neck width>4 mm and/or a fundus/neck ratio<2) were not considered amenable to endovascular coiling for fear that a coil may prolapse into the parent vessel, leading to altered flow dynamics and stroke. More recently, expandable stents have been placed in the parent vessel—acting as a scaffold across the neck of the aneurysm, to prevent coil prolapse. For wide neck aneurysms, this has held promise in preventing coil migration. The introduction of the flexible intracranial stent (Neuroform; Boston Scientific) improved the management of these complex intracranial aneurysms, but was associated stent migration/misplacement and difficulties in coil delivery. In this system, the coils are inserted into the aneurysm dome via the fenestrations in the stent. The limitation with this design is that the fenestrations still allow blood to enter the aneurysm. Thus this strategy depends on the delivery of coils to occlude the dome. The introduction of the coils through the fenestrations in the stent can be associated with dislodgement/migration of the stent during the coiling. More recently, to avoid this problem, intracranial balloons have been used in conjunction with coils and stents. In this technique, the balloon is first inserted into the parent vessel, distal to the aneurysm. Once in place, the balloon is inflated, occluding distal blood flow. During this temporary occlusion, the coils are then inserted and are placed in the dome of the aneurysm. During this process, the balloon is intermittently inflated and deflated—in an attempt to prevent distal ischemia. Finally, once the coiling of the aneurysm is complete, a stent is placed across the neck of the aneurysm to prevent coil prolapse. The balloon is then deflated and distal blood flow resumed.
More recently, further advances have been made for the endovascular treatment of intracranial aneurysms using flow-diversion principles instead of space-occupying principles such as endovascular coiling. One of these devices, the JOSTENT (Abbott), is comprised of an expandable PTFE barrier between two stainless steel stents. Accordingly, it does not have fenestrations and has been used to extensively in cardiac endovascular procedures. This stent is placed in the parent vessel such that it directly occludes the neck of the aneurysm and prevents flow into the aneurysm. Unfortunately, this has had limited applications in the cranial vasculature, as its design is inflexible and difficult to position. In addition, its geometry poses a risk for occlusion of perforating vessels that may be in the vicinity of the aneurysm neck. Another flow-diversion device, the Pipeline Embolization Device (eV3), has been successful in decreasing blood flow into the aneurysm, while maintaining the patency of surrounding branch vessels. The Pipeline is a self-expanding cylindrical braided mesh construct that has decreased porosity size along the entire length of the stent which occludes the neck of the aneurysm. The cells of the mesh are sufficient to embolize the aneurysm while maintaining patency of the parent artery and minimizing disruption to flow into perforating vessels near the aneurysm.
The modern era of aneurysm treatment began with Hunterian ligation of the parent vessel. With increasing sophistication of surgical and endovascular management techniques, the indications for vessel occlusion for management of complex aneurysms are limited. A number of strategies have been devised for arterial occlusion including the Selverstone clamp, a device that allowed for gradual occlusion of the vessel. The rationale for gradual occlusion of the vessel was to promote the capacity for collateral circulation of the circle of Willis, in the hope of preventing post-occlusion cerebral infarction. This device could be reopened at the first sign of cerebral ischemia secondary to insufficient collateral circulation. A number of subsequent devices all incorporated an external mechanism that allowed the surgeon to gradually decrease the caliber of the vessel until ultimately achieving complete occlusion. In complex vascular disorders, deemed untreatable by both direct surgical or endovascular techniques, a test occlusion by balloon is performed by endovascular techniques. Unfortunately this does not provide a gradual occlusion of the vessel and does not allow for the gradual development of collateral circulation. At present, no endovascular technique allows for the gradual occlusion of a vessel.